The present invention relates to the production of solid ribbons and more particularly to a method for producing solid ribbons which utilize gas jets directed approximately at the intersection of the ribbon and the average surface level of the liquid in order to obtain the force needed to balance surface tension.
It has long been recognized that very considerable savings can be effected in the manufacture of many semiconductor devices (transistors, diodes, integrated circuits, and solar cells for the direct conversion of sunlight to electricity) if the semiconductor material could be produced in ribbon form with suitable perfection of lattice and surface. A ribbon is a long length of material whose thickness is very small compared to the width or length; in this case, thickness is meant to be about 0.004-0.012 inch, width about 1 at at least 3 inches, and a length limited by convenience.
The following references present typical techniques used to produce semiconductor (Si or Ge) ribbons. These references contain many other references.
1. "Metallurgy of Elemental and Compound Semiconductors", R. O. Grubel, editor, Vol. 12 of Metallurgical Society Conferences, Boston, Massachusetts, Aug. 1970, Interscience Publishers, N.Y.
2. U.S. Pat. No. 3,162,507 on dendritic growth by S. Dermatis.
3. J. C. Boatman and P. C. Goundry, "Process for Growth of Single Crystal Silicon Ribbon", Electrochemical Technology 5 98 (1967).
4. Don E. Swets, "Growth of Nondendritic Single Crystal Ribbons of Germanium", Electrochemical Technology 5, 385 (1967).
5. "The Growth of Crystals from Liquids", J. C. Brice, North Holland Publishing Co., 1973, p. 246.
6. R. A. Laudise et al "Crystal Growth" in Annual Review of Materials Science, Vol. 1, 1971
7. P. C. Goundry, "Method and Apparatus for Producing Crystalline Semiconductor Ribbon", U.S. Pat. No. 3,453,352, Jul. 1, 1969.
8. B. Chalmers, H. E. LaBelle, Jr., A. I. Mlavsky, Mater. Res. Bull., 6, 681 (1971).
9. B. Chalmers, H. E. LaBelle, Jr., A. I. Mlavsky, J. Cryst. Growth 13/14, 84 (1972).
Other than dendritic growth, two techniques have been used:
1. electromagnetic repulsion to shape the liquid; and,
2. an "orifice" to shape the liquid. Both techniques circumvent the tendency of surface tension to produce a circular cross-section rather than a rectangular (or ribbon) cross-section.
The difficulties with dendritic growth are the existence of the bulky dendrites on either edge of the ribbon, and the non-ideal perfection of the crystal, i.e. twins and high density of dislocations. These reduce the electrical performance of electronic devices made from them. Further, dendritic growth appears to be more critical and expensive than expected.
Electromagnetic repulsion requires large currents for the needed forces and is, therefore, energy intensive. Provision must be made to radiate or otherwise dissipate the joule heat generated in the ribbon at or near the freezing interface. Reference (1) above has a detailed review of this approach. Hence, this technique seems undesirable from a practical viewpoint.
Shaping via an "orifice" appears to be successful, although the control problems are considerable. A representative example of orifice ribbon generation is shown in U.S. Pat. No. 3,453,352. For silicon, the material of the orifice will remain a difficulty because the silicon must "wet" the orifice and silicon liquid is very reactive and "corrosive". Similarly, the die problem in the Tyco Corporation EFG process (edge-defined film-fed growth) is a difficulty (References 8 and 9).
It is accordingly a general object of the invention to provide a method for producing solid (single or polycrystalline, or non-crystalline) ribbons which circumvents the difficulties of the prior art techniques.
It is a specific object of the invention to provide a method for producing solid ribbons through the force generated by a fluid stream or streams directed at a liquid column of the ribbon material.
It is a feature of the invention that the method thereof can be practiced with relatively inexpensive "off-the-shelf" hardward.
It is another feature of the invention that the ribbon configuration can be controlled easily.